1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, such as a printer, a copying machine, and a facsimile machine.
2. Description of the Related Art
In some fixing apparatuses used in image forming apparatuses to fix an unfixed toner image (or unfixed developer image) borne on a recording medium by applying heat thereto to form a permanent fixed image, use is made of a heat roller type fixing system including a fixing member provided with an elastic layer. However, in the heat roller type fixing system having an elastic layer, the heat capacity of the heat roller itself tends to be large. For this reason, it has a drawback that a time required to raise the temperature of the fixing member to a temperature adequate to fix a toner image is long. (The time required to raise the temperature of the fixing member to an adequate temperature will be hereinafter referred to as “warm-up time”.)
In view of the above, in recent years, fixing apparatuses using a film heating system, in which the heat capacity of a fixing member is small, have been increasingly used. In the fixing apparatus using a film heating system, a heat resistant film (which will be referred to as a fixing sleeve or an endless belt) is pinched between a ceramic heater serving as a heat member and a pressure roller serving as a pressurizing member to form a fixing nip portion. In this fixing nip portion, a recording medium on which an unfixed toner image has been formed and borne is introduced between the fixing sleeve and the pressure roller, and the recording medium is pinched and conveyed together with the fixing sleeve. Thus, in the fixing nip portion, heat of the ceramic heater is transferred to the recording medium via the fixing sleeve, whereby the unfixed toner image is fixed as a permanent fixed image on the recording medium with an aid of the pressurizing force in the fixing nip portion. In the case of the above-described fixing apparatus using a film heating system, the heat capacity of the fixing sleeve that serves as the fixing member is small, and therefore the warm-up time can be made shorter. On the other hand, however, the fixing apparatus using a film heating system encounters a problem in terms of heat conduction in the direction (which will be hereinafter referred to as the longitudinal direction) perpendicular to the direction of conveyance of the recording medium, and the non-sheet passing portion temperature rise that will be described later will occur in this system.
Specifically, the heater or the heat member has an electrified heat generation resistive layer extending along the longitudinal direction. The electrified heat generation resistive layer is electrified (i.e. supplied with electrical power) through the electrodes provided at the ends thereof, whereby it generates a specific quantity of heat per unit length. The length of the electrified heat generation resistive layer along the longitudinal direction is designed to be large enough to enable fixation of the end or edge portions of a recording medium having the maximum width that can be supplied to (or passed through) the image forming apparatus. Thus, the electrified heat generation resistive layer generates heat throughout its entire length along the longitudinal direction irrespective of the width of recording mediums that are passed. For example, in the case of an image forming apparatus in which the maximum width size of the usable recording mediums is the letter size width (LTR width), when a recording medium having a width not larger than A4 size, which is smaller than letter size, passes, the following situation will occur. That is, heat will accumulate in the area outside the recording medium passing area (which outside area will be hereinafter referred to as the “non-sheet passing portion”), since removal of heat energy by the recording medium will not occur in the non-sheet passing portion. This phenomenon is the non-sheet passing portion temperature rise.
In recent years, with increases in the speed of operation of the image forming apparatus, the recording sheet conveying speed in the fixing apparatus has become very high. Therefore, the temperature of the heater serving as a heat member is very high in order to give heat to the recording medium to ensure fixability of the toner image on the recording medium. Consequently, the non-sheet passing portion temperature rise is more likely to occur.
If the temperature rise in the non-sheet passing portion is large, so-called high temperature offset image errors occur in some cases. In addition, the fixing sleeve may be thermally affected, and there arise problems such as deterioration of durability thereof in some cases.
In view of the above, according to a widely known method, when the temperature of the non-sheet passing portion reaches a predetermined temperature, the printing operation is suspended for a certain period of time until the temperature of the non-sheet passing portion falls. According to another method, when recording mediums are supplied or conveyed successively, the interval between the trailing edge of a preceding recording medium and the leading edge of the succeeding recording medium is extended to reduce the temperature rise in the non-sheet passing portion. However, these methods are disadvantageous in that the throughput (i.e. the number of sheets processed in the image formation process per unit time) is greatly decreased, which leads to a decrease in the productivity.
Japanese Patent Application Laid-Open No. H05-135848 discloses a fixing apparatus having features described in the following. In this apparatus, a plurality of temperature detection elements are provided along the longitudinal direction of a heater. At least one of the temperature detection elements is a first temperature detection element provided in the area in which recording mediums of all the sizes pass, and at least one other temperature detection element is a second temperature detection element provided in an area that becomes a non-sheet passing portion when recording mediums having a certain size(s) pass. When a small size sheet, or a recording medium having a size that does not extend to the position at which the second temperature detection element is provided, is supplied, power supply to the heater is controlled normally so that the output value of the first temperature detection element is kept constant. When it is detected that the temperature detected by the second temperature detection element or the temperature at the non-sheet passing portion reaches a predetermined temperature while power supply to the heater is controlled so that the output value of the first temperature detection element is kept constant, the control is switched into a control for keeping the output value of the second temperature detection element constant. The fixing apparatus disclosed in this document can prevent an unduly large temperature rise in the non-sheet passing portion by the above-described control.
The above described prior art system is advantageous in that an unduly large temperature rise in the non-sheet passing portion of the heater can be prevented by switching the power supply control into the control for keeping the output value of the second temperature detection element constant, when the second temperature detection element disposed at a position in the non-sheet passing portion detects that a predetermined temperature is reached.
However, the control for keeping the temperature in the non-sheet passing portion constant ha the following disadvantage in some cases.
As is the case with the above described prior art, power supply to the heater is normally controlled so that the output value of a temperature detection element provided in the central portion of the recording medium passing area with respect to the longitudinal direction (i.e. the first temperature detection element) is kept constant. In this case, in the time period during which a recording medium is passing through the fixing nip portion (which will be hereinafter described as “during sheet passing”), the power supplied to the heater is made large, because heat is removed by the recording medium. On the other hand, in the time period after a preceding recording medium has left the fixing nip portion and before the succeeding recording medium enters the fixing nip portion (which will be hereinafter described as “during sheet interval), removal of heat by the recording medium does not occur. For this reason, in a case where a control for keeping the temperature constant is performed, the power supplied to the heater during sheet interval is controlled to be smaller than that during sheet passing. In this case, if recording mediums pass successively, the temperature at the non-sheet passing portion, in which heat is not removed by the recording medium at any time, rises steeply during sheet passing during which a large power is supplied, and falls, conversely, during sheet interval during which the supplied power is made smaller. FIG. 8 schematically illustrates the temperature at the central portion of the sheet passing area, the temperature at the non-sheet passing portion, and the power supplied or input to the heater in such a case. FIG. 8 schematically illustrates the temperature at the central portion of the sheet passing area (or the temperature at the sheet passing portion) by the solid line, the temperature at the non-sheet passing portion by the broken line, and the input power (or the input power ratio) by the alternate long and short dashed line, in a case where a control for keeping the temperature of the sheet passing portion constant is performed (during successive sheet passing).
The magnitude of such changes in the temperature at the non-sheet passing portion increases with increases in the heat capacity, basis weight and thickness of the recording medium.
In a case where power supply to the heater is controlled so that the temperature at the non-sheet passing portion is kept constant when the temperature at the non-sheet passing portion has risen to a predetermined temperature, the temperature at a portion in the sheet passing area will behave as follows. In contrast with the case where power supply to the heater is controlled so that the temperature at a portion in the sheet passing area is kept constant, the temperature at the sheet passing portion decreases abruptly during sheet passing, because heat is removed by the recording medium. FIG. 9 schematically illustrates the temperature at the sheet passing portion, the temperature at the non-sheet passing portion, and the power supplied or input to the heater in relation to the number of passing sheets, in a case where a control for keeping the temperature at the non-sheet passing portion constant is performed. FIG. 9 schematically illustrates the temperature at the sheet passing portion by the solid line, the temperature at the non-sheet passing portion by the broken line, and the input power (or the input power ratio) by the alternate long and short dashed line, in a case where a control for keeping the temperature at the non-sheet passing portion constant is performed after the timing indicated by X. As will be understood from the temperature at the sheet passing portion indicated by arrows in the left portion of FIG. 9, the quantity of heat given to a sheet of the recording medium is larger toward leading edge of the recording medium and smaller toward the trailing edge thereof. Thus, the temperature falls as the sheet passes. Consequently, in one sheet of recording medium, the glossiness of the image may decrease toward the trailing edge, and the fixability of the image may decrease toward the trailing edge.